r+q = 10i+10j, so |r+q| = 10√2 x^3 + y^3 - 2xy = 11 3x^2 + 3y^2 y' = -2y -2xy' y' = (3x^2 - 2y)/(2x - 3y^2) so y' at (2,1) = (3*4 - 2*1)/(2*2 - 3*1) = 10
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